Nucleoside analogs are an important class of compounds that are useful in the treatment of disease. For example, nucleoside analogs have been used in the treatment of cancers and viral infections. After entry into a cell, nucleoside analogs are frequently phosphorylated by nucleoside salvage pathways in which the analogs are phosphorylated to the corresponding mono-, di-, and triphosphates. Among other intracellular destinations, triphosphorylated nucleoside analogs often serve as substrates for DNA or RNA polymerases and become incorporated into DNA and/or RNA. Where triphosphorylated nucleoside analogs are strong polymerase inhibitors, they may induce premature termination of a nascent nucleic acid molecule. Where triphosphorylated nucleoside analogs are incorporated into nucleic acid replicates or transcripts, gene expression or disruption of function may result.
Some nucleoside analogs may be efficacious because of their ability to inhibit adenosine kinase. Adenosine kinase catalyzes the phosphorylation of adenosine to adenosine 5′-monophosphate (AMP). Inhibition of adenosine kinase may effectively increase the extracellular level of adenosine in humans and thereby serve as a treatment of ischemic conditions such as stroke, inflammation, arthritis, seizures, and epilepsy.
The last few decades have seen significant efforts expended in exploring therapeutic uses of nucleoside analogs. For example, certain pyrimido[4,5-d]pyrimidine nucleosides are disclosed in U.S. Pat. No. 5,041,542 to Robins et al. as being effective in treatment against L1210 in BDF1 mice. Additionally, 3-β3-D-ribofuranosylthiazolo[4,5-d]pyrimidines demonstrating significant immunoactivity, including murine spleen cell proliferation and in vivo activity against Semliki Forest virus, are disclosed U.S. Pat. Nos. 5,041,426 and 4,880,784 to Robins et al. A number of publications have also described non-glycosyl derivatives of the thiazolo[4,5-d]pyrimidine moiety. See, e.g., U.S. Pat. Nos. 5,994,321 and 5,446,045; Revankar et al., J. HET. CHEM., 30, 1341-49 (1993); Lewis et al., J. J. HET. CHEM., 32, 547-56 (1995).
3,5-Disubstituted-3H-thiazolo[4,5-d]pyrimidin-2-one compounds have been shown to have immunomodulatory activity. The preparation and usefulness of this class of compounds is discussed in U.S. Application Publication No. US2006/0160830 (U.S. application Ser. No. 11/304,691), which is incorporated herein by reference in its entirety. This application describes the synthesis of the free base compound 5-amino-3-(2′-O-acetyl-3′-deoxy-beta-D-ribofuranosyl)-3H-thiazolo[4,5-d]pyrimidin-2-one. The purity of this compound may vary based on purification methods due to the amorphous nature of the free base. Adequate purification of the free base may be limited to the use of certain solvents which are not acceptable for human consumption. In addition, the amorphous form (free base) of this compound tends to be hydroscopic which may make the compound susceptible to hydrolysis. Accordingly, a method for producing a crystalline form of this compound with high purity and stability having low amounts of non-toxic solvent is desirable for pharmaceutical applications.